The invention relates to a neutron and/or gamma radiation detecting system with a large reception area for the radiation to be detected, which radiation generates in scintillation materials light energy that is coupled into fiber light conductors. The system is especially suitable for use in connection with revolving door safety locks.
Neutron detection is utilized as a supplementary measure in connection with lock structures for the detection of unauthorized passage of nuclear materials. This measure increases the chances of detecting the transfer of nuclear materials out of controlled areas without permission since measures adapted to avoid detection of neutron radiation are substantially more involved than measures which do not need consider the detection of .gamma. radiation.
ZnS(Ag) is one of the compounds with the highest scintillation available for the purpose of neutron detection. However, in order to provide an effective detector, efficient light collection and its transmission from the scintillator to the photomultiplier are necessary. The light collector arrangements utilized so far are as follows:
In a publication K. H. Sun, P. R. Malmberg, F. A. Pecjak, "Scintillation Counters", Nucleonis (July 1956), a detector disc is mounted directly on the photomultiplier (PTM), the detector disc being disposed in the entrance window for the radiation. In this arrangement the light transmission path is short and transmission losses are therefore small. A disadvantage, however, is that the coupling cross-section is limited by the size of the window of the photomultiplier utilized. In another arrangement as disclosed in publication W. S. Emmerich, "A Fast Neutron Szintillator", Rev. Scien. Inst., Vol. 25, No. 1, Jan. 1954, a substantially larger usable coupling cross-section (five times larger) is obtained by the use of light guide plates integrated into the scintillator but these units can be built only in small sizes with only a small angle of reception which would cover only a small part of a revolving door chamber, for example.
The disadvantages of light conductors as they are presently used are:
Scattering losses in the contact area between light conductors and the optically denser scintillator materials.
Attenuation losses (light absorption in the necessary transmission length).
Unfavorable surface ratios between light conductor cross-section and collector surface (decoupling area).
The decoupling efficiency of larger scintillator crystals, dependent on the index of refraction, is in the range of 5-11%.
The values given for the coupling and transmission efficiencies of isotropic light conductors (with interface areas with material of optically low density, i.e., air) show that, for scintillators with large surface areas, suitable optical systems have to comply with the following requirements:
Decoupling of light source and light conductor by interposing an optical intermediate layer with an index of refraction which is smaller than that of
the scintillator (n.apprxeq.1.5) so as to avoid scattering losses at the interface.
Coupling of a light intensity sufficient for the transmission distance between the scintillator and the photomultiplier.
Feeding of the light into the fibers through face areas is utilized in connection with low level light amplifiers and large display areas as shown in a publication W. B. Allan, "Fibre Optics, Theory and Practice", Plenum Press, London, NY, 1973--in reverse of a detector application. A disadvantage, however, is a need for a relatively large amount of fiber material and a relatively low reduction factor achievable for the coupling-in surface/uncoupling surface ratio.
The object of the presnt invention is to provide a large area detection system of the type described which, however, permits utilization of an effective scintillation material and also to take advantage of the properties of fibers for the transmission of light to the detector systems.